Cancer remains to be one of the most deadly threats to human health. In the U.S., cancer affects nearly 1.3 million new patients each year, and is the second leading cause of death after heart disease, accounting for approximately 1 in 4 deaths. It is also predicted that cancer may surpass cardiovascular diseases as the number one cause of death within 5 years. Solid tumors are responsible for most of those deaths. Although there have been significant advances in the medical treatment of certain cancers, the overall 5-year survival rate for all cancers has improved, only by about 10% in the past 20 years. Cancers, or malignant tumors, metastasize and grow rapidly in an uncontrolled manner, making timely detection and treatment extremely difficult. Furthermore, cancers can arise from almost any tissue in the body through malignant transformation of one or a few normal cells within the tissue, and each type of cancer with particular tissue origin differs from the others.
Current methods of cancer treatment are relatively non-selective. Surgery removes the diseased tissue; radiotherapy shrinks solid tumors; and chemotherapy kills rapidly dividing cells. Chemotherapy, in particular, results in numerous side effects, in some cases so severe as to limit the dosage that can be given and thus preclude the use of potentially effective drugs. Moreover, cancers often develop resistance to chemotherapeutic drugs.
Thus, there is an urgent need for specific and more effective cancer therapies.
Angiogenesis is an important cellular event in which vascular endothelial cells proliferate, prune and reorganize to form new vessels from preexisting vascular network. There are compelling evidences that the development of a vascular supply is essential for normal and pathological proliferative processes (Folkman and Klagsbrun (1987) Science 235:442-447). Delivery of oxygen and nutrients, as well as the removal of catabolic products, represent rate-limiting steps in the majority of growth processes occurring in multicellular organisms. Thus, it has been generally assumed that the vascular compartment is necessary, not only for organ development and differentiation during embryogenesis, but also for wound healing and reproductive functions in the adult.
Angiogenesis is also implicated in the pathogenesis of a variety of disorders, including but not limited to, tumors, proliferative retinopathies, age-related macular degeneration, rheumatoid arthritis (RA), and psoriasis. Angiogenesis is essential for the growth of most primary tumors and their subsequent metastasis. Tumors can absorb sufficient nutrients and oxygen by simple diffusion up to a size of 1-2 mm, at which point their further growth requires the elaboration of vascular supply. This process is thought to involve recruitment of the neighboring host mature vasculature to begin sprouting new blood vessel capillaries, which grow towards, and subsequently infiltrate, the tumor mass. In addition, tumor angiogenesis involve the recruitment of circulating endothelial precursor cells from the bone marrow to promote neovascularization. Kerbel (2000) Carcinogenesis 21:505-515; Lynden et al. (2001) Nat. Med. 7:1194-1201.
While induction of new blood vessels is considered to be the predominant mode of tumor angiogenesis, recent data have indicated that some tumors may grow by co-opting existing host blood vessels. The co-opted vasculature then regresses, leading to tumor regression that is eventually reversed by hypoxia-induced angiogenesis at the tumor margin. Holash et al. (1999) Science 284:1994-1998.
In view of the remarkable physiological and pathological importance of angiogenesis, much work has been dedicated to the elucidation of the factors capable of regulating this process. It is suggested that the angiogenesis process is regulated by a balance between pro- and anti-angiogenic molecules, and is derailed in various diseases, especially cancer. Carmeliet and Jain (2000) Nature 407:249-257.
Vascular endothelial cell growth factor (VEGF), which is also termed VEGF-A or vascular permeability factor (VPF), has been reported as a pivotal regulator of both normal and abnormal angiogenesis. Ferrara and Davis-Smyth (1997) Endocrine Rev. 18:4-25; Ferrara (1999) J. Mol. Med. 77:527-543. Compared to other growth factors that contribute to the processes of vascular formation, VEGF is unique in its high specificity for endothelial cells within the vascular system. VEGF is essential for embryonic vasculogenesis and angiogenesis. Carmeliet et al. (1996) Nature 380:435-439; Ferrara et al. (1996) Nature 380:439-442. Furthermore, VEGF is required for the cyclical blood vessel proliferation in the female reproductive tract and for bone growth and cartilage formation. Ferrara et al. (1998) Nature Med. 4:336-340; Gerber et al. (1999) Nature Med. 5:623-628.
In addition to being an angiogenic factor in angiogenesis and vasculogenesis, VEGF, as a pleiotropic growth factor, exhibits multiple biological effects in other physiological processes, such as endothelial cell survival, vessel permeability and vasodilation, monocyte chemotaxis and calcium influx. Ferrara and Davis-Smyth (1997), supra. Moreover, recent studies have reported mitogenic effects of VEGF on a few non-endothelial cell types, such as retinal pigment epithelial cells, pancreatic duct cells and Schwann cells. Guerrin et al. (1995) J. Cell Physiol. 164:385-394; Oberg-Welsh et al. (1997) Mol. Cell. Endocrinol. 126:125-132; Sondell et al. (1999) J. Neurosci. 19:5731-5740.
Substantial evidence also implicates VEGF's critical role in the development of conditions or diseases that involve pathological angiogenesis. The VEGF mRNA is overexpressed by the majority of human tumors examined (Berkman et al. J Clin Invest 91:153-159 (1993); Brown et al. Human Pathol. 26:86-91 (1995); Brown et al. Cancer Res. 53:4727-4735 (1993); Mattern et al. Brit. J. Cancer. 73:931-934 (1996); and Dvorak et al. Am J. Pathol. 146:1029-1039 (1995)). Also, the concentration of VEGF in eye fluids are highly correlated to the presence of active proliferation of blood vessels in patients with diabetic and other ischemia-related retinopathies (Aiello et al. N. Engl. J. Med. 331:1480-1487 (1994)). Furthermore, recent studies have demonstrated the localization of VEGF in choroidal neovascular membranes in patients affected by AMD (Lopez et al. Invest. Ophtalmo. Vis. Sci. 37:855-868 (1996)).
Given its central role in promoting tumor growth, VEGF provides an attractive target for therapeutic intervention. Indeed, a variety of therapeutic strategies aimed at blocking VEGF or its receptor signaling system are currently being developed for the treatment of neoplastic diseases. Rosen (2000) Oncologist 5:20-27; Ellis et al. (2000) Oncologist 5:11-15; Kerbel (2001) J. Clin. Oncol. 19:45S-51S. So far, VEGF/VEGF receptor blockade by monoclonal antibodies and inhibition of receptor signaling by tyrosine kinase inhibitors are the best studied approaches. VEGFR-1 ribozymes, VEGF toxin conjugates, and soluble VEGF receptors are also being investigated.
The anti-VEGF antibody “Bevacizumab (BV)”, also known as “rhuMAb VEGF” or “Avastin™”, is a recombinant humanized anti-VEGF monoclonal antibody generated according to Presta et al. (1997) Cancer Res. 57:4593-4599. It comprises mutated human IgG1 framework regions and antigen-binding complementarity-determining regions from the murine anti-hVEGF monoclonal antibody A.4.6.1 that blocks binding of human VEGF to its receptors. Approximately 93% of the amino acid sequence of Bevacizumab, including most of the framework regions, is derived from human IgG1, and about 7% of the sequence is derived from the murine antibody A4.6.1. Bevacizumab has a molecular mass of about 149,000 daltons and is glycosylated. Bevacizumab is being investigated clinically for treating various cancers, and some early stage trials have shown promising results. Kerbel (2001) J. Clin. Oncol. 19:45S-51S; De Vore et al. (2000) Proc. Am. Soc. Clin. Oncol. 19:485a; Johnson et al. (2001) Proc. Am. Soc. Clin. Oncol. 20:315a; Kabbinavar et al. (2003)J. Clin. Oncol. 21:60-65.